1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display device.
2. Description of Related Art
Recently, liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics are used in office automation equipment and video units and the like. Such LCDs typically use a liquid crystal (LC) interposed between upper and lower substrates with an optical anisotropy. Since the LC has thin and long LC molecules, the alignment direction of the LC molecules can be controlled by applying an electric field to the LC molecules. When the alignment direction of the LC molecules is properly adjusted, the LC is aligned and light is refracted along the alignment direction of the LC molecules to display images.
In general, LCD devices are divided into transmissive LCD devices and reflective LCD devices according to whether the display device uses an internal or external light source.
A conventional transmissive LCD device includes an LCD panel and a backlight device. The incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The transmissive LCD device requires a high, initial brightness, and thus electrical power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device, and the battery can not be used for a lengthy period of time.
In order to overcome the problems described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light instead of the backlight by using a reflective opaque material as a pixel electrode, it is light and easy to carry. In addition, the power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used as an electric diary or a PDA (personal digital assistant).
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that of the outdoors. Therefore, the reflective LCD device can not be used where the ambient light is weak or does not exist. In order to overcome the problems described above, a transflective LCD device has been researched and developed. The transflective LCD device can be transferred according to the user's selection from the transmissive mode to the reflective mode, or vise versa.
FIG. 1 is a schematic perspective view of a conventional transflective LCD device 11.
In FIG. 1, the conventional transflective LCD device 11 includes upper and lower substrates 15 and 21 with an interposed liquid crystal 23. The upper and lower substrates 15 and 21 are sometimes respectively referred to as a color filter substrate and an array substrate. On a surface facing the lower substrate 21, the upper substrate 15 includes a black matrix 16 and a color filter layer 18. The color filter layer 18 includes a matrix array of sub-color filters 17 of red (R), green (G), and blue (B) that are formed such that each color filter is bordered by the black matrix 16. The upper substrate 15 also includes a common electrode 13 over the color filter layer 18 and over the black matrix 16. On a surface facing the upper substrate 15, the lower substrate 21 includes an array of thin film transistors (TFTs) “T” that act as switching devices. The array of TFTs is formed to correspond with the matrix of color filters. A plurality of crossing gate and data lines 25 and 27 are positioned such that a TFT is located near each crossing of the gate and data lines 25 and 27. The lower substrate 21 also includes a plurality of pixel electrodes 19, each in an area defined between the gate and data lines 25 and 27. Such areas are often referred to as pixel regions “P.” Each pixel electrode 19 includes a transmissive portion “A” and a reflective portion “C”. The transmissive portion “A” is usually formed from a transparent conductive material having a good light transmittance, for example, indium-tin-oxide (ITO). Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion “C”.
FIG. 2 is a schematic cross-sectional view of a conventional transflective LCD device such as the device 11 of FIG. 1.
In FIG. 2, upper and lower substrates 15 and 21 are facing and spaced apart from each other and a liquid crystal layer 23 is interposed therebetween. A backlight apparatus 45 is disposed over the outer surface of the lower substrate 21. On the inner side of the upper substrate 15, a color filter layer 18 for passing only the light of a specific wavelength and a common electrode 14 functioning as one electrode for applying a voltage to the liquid crystal layer 23 are subsequently formed. On the inner surface of the lower substrate 21, a pixel electrode 32 functioning as the other electrode for applying a voltage to the liquid crystal layer 23, a passivation layer 34 having a transmissive hole 31 exposing a portion of the pixel electrode 32, and a reflective plate 36 are subsequently formed. An area corresponding to the reflective plate 36 is a reflective portion “C” and an area corresponding to the portion of the pixel electrode 32 exposed by the transmissive hole 31 is a transmissive portion “A”.
A cell gap “d1” at the transmissive portion “A” is about twice of a cell gap “d2” at the reflective portion “C” to reduce the light path difference. A retardation “Δn·d” of the liquid crystal layer 23 is defined by a multiplication of refractive index anisotropy “Δn” with a cell gap “d” and the light efficiency of the LCD device is proportional to the retardation. Therefore, to reduce the difference of light efficiencies between the reflective and transmissive modes, the retardations of the liquid crystal layer 23 at two portions should be nearly equal to each other by making the cell gap of the transmissive portion larger than that of the reflective portion.
However, even though the light efficiencies of the liquid crystal layer between the reflective and transmissive modes become equal by making the cell gaps different, the light passing the color filters at different locations is different so that the brightness can be different at the front of the display device. The transmittance of the color filter resin whose absorption coefficient is high for a specific wavelength and low for the other wavelengths has the following relation considering only the absorption, i.e., the transmittance is inversely proportional to the absorption coefficient and the distance that light passes:T=exp(−α(λ)d)where T is transmittance, α(λ) is an absorption coefficient depending on the wavelength and d is a distance that light passes.
Since the color filter resin is a viscous material, the thickness of the color filter resin is hard to control and the color filter layer can not be made less than a specific thickness. Therefore, the color filter layers of the reflective and transmissive portions have the same thickness and the different absorption coefficient (i.e., different material) for the uniform transmittance.
However, if the color filter layers of the reflective and transmissive portions are formed of different materials, the process and the cost would be increased and the yield would be decreased.
To solve the above problems, a fabricating method of the color filter layers with the same resin is suggested. In this method, the color filter layers at the reflective and transmissive portions have the same absorption coefficient but a different thickness so that the transmittance has the same value.
FIGS. 3A and 3B are transmittance spectrums of first and second red color filter layers for the reflective mode having a specific thickness and two times the specific thickness, respectively.
Generally, a visible light has a wavelength ranging about 400 to 700 nanometers. Red (R), green (G) and blue (B) colors roughly correspond to wavelengths of 650, 550 and 450 nanometers, respectively.
In FIG. 3A, the transmittances at wavelengths corresponding to R, G and B are about 97%, 20% and 58%, respectively. Even though the transmittance for red color is high, the transmittances for the other colors are also not negligible so that a satisfying color purity is not obtained.
In FIG. 3B, since the second red color filter layer has twice the thickness and square transmittance compared with the first red color filter layer of FIG. 2A, the transmittances at wavelengths corresponding to R, G and B are about 94%, 4% and 34%, respectively. Although the transmittance is decreased for all colors, the decreased amount is different for the individual colors, for example, about 5%, 16% and 24% for R, G and B, respectively.
Therefore, the color purity of the second red color filter layer is improved and this result can be applied for the green and blue color filters so that the transmittance and color purity of the transflective LCD device using the same kind of color filter resin can be uniform for the reflective and transmissive portions.
A transflective LCD device using a dual thickness color filter (DCF) of the above-mentioned principle is suggested in Korean Patent Application No. 2001-9979 of the applicant.
FIG. 4 is a cross-sectional view of a transflective LCD device using the DCF according to a related art.
In FIG. 4, a transparent buffer layer 64 is formed on the inner surface of the upper substrate 15 only at a reflective portion “C”, and a color filter layer 62 is formed on the entire upper substrate 15. Therefore, the color filter layer 62 of a transmissive portion “A” is thicker than that of the reflective portion “C” so that the color purity of the transmissive portion “A” can be improved. The transparent buffer layer 64 is formed by depositing and patterning one of an insulating material group comprising acrylic resin, benzocyclobutene (BCB) and silicon nitride (SiNx). Therefore, the buffer layer 64 of a yellowish color is not perfectly transparent and the transmittance of the buffer layer 64 is lower than that of glass substrate. Moreover, since light is partially reflected at the interface between the buffer layer 64 and the substrate 15, the transmittance at the reflective portion “C” is more decreased.
FIGS. 5A and 5B are cross-sectional views of color filter substrates using the DCF having transparent buffer layers of first and second thicknesses, respectively, according to a related art.
In FIG. 5A, the substrate 15 has a transmissive portion “A” and a reflective portion “C”. A black matrix 70 and a transparent buffer layer 64 are formed in the reflective portion “C” and a color filter layer 62 is formed on the entire surface of the substrate 15. Since the transparent buffer layer 64 of a first thickness has a low step 52 at the borderline of the transmissive portion “A” and the reflective portion “C” so that the surface of the color filter layer 62 can be planarized. Moreover, since the color filter layer 62 at the transmissive portion “A” is thicker than that at the reflective portion “C”, the color purity can be improved at the transmissive portion “A”. However, since the thickness of the transparent buffer layer 64 has a limit for the planarization of the color filter layer 62, the thickness ratio of the color filter layer 62 also has a limit and the improvement of the color purity is limited.
In FIG. 5B, to have a desired thickness ratio of the color filter layer 62, the transparent buffer layer 64 has a second thickness higher than the first thickness of FIG. 5A and a high step 54 at the borderline of the transmissive portion “A” and the reflective portion “C”. Since the color filter layer 64 is made of a viscous resin and formed according to the surface of the underlayer, the color filter layer 64 also has a step 55 at the top surface. Therefore, the difference “Δd” between the designed thickness d3 and the fabricated thickness d4 occurs and the improvement of the color purity of the transmissive portion “A” is limited.
Generally, the thickness of a conventional color filter layer for the reflective LCD device is controlled to have the average transmittance in the range of about 55 to 70%. If the thickness of the color filter layer is increased, the transmittance and the color appearance of the color filter layer are varied. For the color filter layer twice as thick as the conventional color filter, the transmittance and the color appearance are 46% and 24.9%, respectively. On the other hand, for the color filter layer 1.3 times as thick as the conventional color filter, the transmittance and the color appearance are 54.7% and 14.1%, respectively. Consequently, if the color filter layer of the transmissive portion is not formed with a desired thickness, the color property of the transmissive portion can not approach that of the reflective portion.
Furthermore, since the step of the color filter layer also degrades the planarization property of the common electrode on the color filter layer, the display quality of conventional LCDs is degraded.